A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In an immersion apparatus, immersion fluid is handled by a fluid handling system, device structure or apparatus. In an embodiment the fluid handling system may supply immersion fluid and therefore be a fluid supply system. In an embodiment the fluid handling system may at least partly confine immersion fluid and thereby be a fluid confinement system. In an embodiment the fluid handling system may provide a barrier to immersion fluid and thereby be a barrier member, such as a fluid confinement structure. In an embodiment the fluid handling system may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid so the fluid handling structure may be referred to as a seal member; such a seal member may be a fluid confinement structure. In an embodiment, immersion liquid is used as the immersion fluid. In that case the fluid handling system may be a liquid handling system. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.
Handling immersion liquid in a lithographic apparatus brings with it one or more problems of liquid handling. A gap normally exists between an object, (e.g., a substrate and/or a sensor) and a table (e.g. a substrate table or a measurement table) around the edge of the object. U.S. patent application publication U.S. 2005-0264778, discloses filling that gap with material or providing a liquid source or low pressure source to deliberately fill the gap with liquid in order to avoid bubble inclusion as the gap passes under the liquid supply system and/or to remove any liquid which does enter the gap.
Liquid from a gap between an edge of an object and a table on which the object is positioned may be removed through a channel. When the liquid is removed, gas may also be removed through the channel. A two-phase flow of liquid and gas may pass through the channel. Such a channel for two-phase flow may be present in other parts of the lithographic apparatus, for example in a liquid confinement system. Such a two-phase flow can lead to undesirable temperature changes in the component that the channel is formed in, or other nearby components. Temperature changes and/or any other load imposed by the two-phase flow on an object containing such a channel will induce deformation of the object.
In an embodiment, the lithographic apparatus comprises one or more objects that need to be positioned, e.g. a substrate table or a support structure. In an embodiment a specific point, referred to as the point of interest, on the object needs to be positioned accurately. For example, the point of interest may be a target portion on the substrate. However, it may not be possible to directly measure the position of the point of interest. In an embodiment, the position of another point, referred to as the measured point, is measured. The position of the point of interest relative to the measured point is assumed. The point of interest and the measured point may be on the same object. Alternatively, the point of interest and the measured point may be on different objects. For example, the point of interest may be on the object and the measured point may be on an object table, the object table being used to carry the object. Temperature variation and/or any other load imposed can cause deformation of the object and/or the object table leading to changes in the position of the point of interest relative to the measured point. As such, the deformation may cause errors in the positioning of the point of interest.
The same principle applies to many objects and object tables in the lithographic apparatus, including components for measuring displacement of parts of the lithographic apparatus. Any deformation in measuring components will lead to inaccurate measurements which may change the position to which a point of interest is moved which may cause errors in the positioning of the point of interest.